15: Molarity, pH Scale and Strengths of Acids and Alkalis Flashcards
What is molarity?
Molarity is the number of moles of solute dissolved in one cubic decimetre of solution.
What is the colour change when litmus solution is added to an acid?
It turns from purple to red.
What is the colour change when litmus solution is added to an alkali?
It turns from purple to blue.
What is the colour change of blue litmus paper in an acid?
It turns from blue to red.
What is the colour change of red litmus paper in an alkali?
It turns from red to blue.
What is the colour of methyl orange indicator in dilute hydrochloric acid?
red
What is the colour of phenolphthalein in dilute hydrochloric acid?
colourless
What is the colour of methyl orange indicator in dilute sodium hydroxide solution?
yellow
What is the colour of phenolphthalein in dilute sodium hydroxide solution?
pink
What is the definition of pH?
pH = -log[H+(aq)]
How is the concentration of hydrogen ions calculated with given pH?
[H+(aq)] = 10-pH
What is the range of the pH scale?
from 0 to 14
Are pH values lower than 0 or higher than 14 possible?
Yes, they are possible for very concentrated acid or alkaline solutions.
What is the pH range of an acidic solution?
pH < 7
What is the pH range of an alkaline solution?
pH > 7
What is the pH range of a neutral solution?
pH = 7
When does [H+(aq)][OH-(aq)] = 10-14 hold?
At 25ºC
Why does pure water have a pH of 7 at 25ºC?
At 25ºC, we have [H+(aq)][OH-(aq)] = 10-14. In addition, water dissociates into hydrogen ions and hydroxide ions in a 1:1 mole ratio. Therefore, [H+(aq)] = 10-7, thus water has a pH of 7 at 25ºC.
Is the pH of boiling pure water the same as that as pure water at 25ºC?
No, the pH of boiling pure water is lower, however, boiling pure water is still neutral.
List 4 methods to measure the pH of a solution.
- Using universal indicator solution.
- Using pH paper.
- Using a pH meter.
- Using a pH sensor connected to a data-logger.
What is the definition of strong acid?
A strong acid is an acid that dissociates almost completely in water to give hydrogen ions.
What is the definition of weak acid?
A weak acid is an acid that dissociates partially in water to give hydrogen ions.
Which type of arrow is used for the dissociation of hydrochloric acid in water?
Single arrow, since all the molecules dissociate into hydrogen ions and chloride ions.
Which type of arrow is used for the dissociation of ethanoic acid in water?
Double arrow, since not all the molecules dissociate into hydrogen ions and ethanoate ions.
List 3 methods to compare the strengths of acids.
- Measuring the pH value of the acids in the same concentration.
- Measuring the electrical conductivity of the acids in the same concentration.
- By comparing the rate of reaction of the acids with metals in the same concentration.
Why does a strong acid have better electrical conductivity than weak acids?
The electrical conductivity of a solution is proportional to the concentration of mobile ions. Strong acids dissociate more compared to weak acids, so the concentration of mobile ions is higher. Thus, they have better electrical conductivity.
Why does a strong acid have a higher rate of reaction with magnesium than weak acids?
Strong acids have a higher concentration of hydrogen ions, thus they react more rapidly than weak acids with magnesium to form hydrogen gas.
Magnesium ribbons is added to two test tubes of strong acid and weak acid of the same concentration. Do the two test tubes produce the same amount of hydrogen gas?
Yes. Although the dissociation of weak acids is incomplete, the hydrogen ions are being consumed in the traction with the magnesium, and the reaction is pushed to completion. Eventually, all hydrogen atoms in the weak acid will be released as hydrogen ions and reacted to make hydrogen gas.
What is the definition of strong alkali?
A strong alkali is an alkali that dissociates almost completely in water to give hydroxide ions.
What is the definition of weak alkali?
A weak alkali is an alkali that dissociates partially in water to give hydroxide ions.
